Magnetron type mass spectrometer



Dec. 22, 1964 w. s. KREISMAN 3,152,760

MAGNETRON TYPE MASS SPECTROMETER Filed Nov. 7, 1961 4 Sheets-Sheet l Flyi [NV ENT OR Walla ce S.Kreism.21,

BY KZWL ATTORNEY 6 1964 yv. s. KREISMAN 3,162,760 MAGNETRON TYPE MASS SPECTROMETER Filed Nov. 7, 1961 4 Sheets-Sheet 2 0- 6. 8 VOL T5 D/FFt'RH/T/AL AMPLIFIER DETZ'CTOR I I INVENTOR Wallace szKrei-sman,

ATTORNEYS Dec. 22, 1964 W. S. KRE ISMAN MAGNETRON TYPE MASS SPECTROMETER Filed Nov. 7, 1961 4 Sheets-$heet 3 DlFFfRENT/AL AND DETECTOR INVENTOR. "(211d ca 6: Krezmzz,

BY I

A T TORNE Y 6' Dec. 22, 1964 w. s. KREISMAN 3,162,760

MAGNETRON TYPE MASS SPECTROMETER Filed Nov. 7, 1961 4 Sheets-Sheet 4 INVENTOR Wallace 5.1(18i8m3n,

BY 9 33M 10 5% ATTORNEYS United States Patent Ofifice 3,162,760 MAGNETRGN TYPE MASS SPECTROMETER Wallace S. Kreisman, Maiden, Mass, assignor to Geophysics Corporation of America, Bedford, Mass a corporation of Delaware] Filed Nov. 7, 1951, Ser. No. 150,691 19 Ciaims. (Cl 250-419) This invention relates to a mass spectrometer and more particularly to a mass spectrometer of the magnetic type which is capable of measuring the presence of preselected ions even though they are intermingled with other ions of varying mass.

In the prior use of mass spectrometers, it has been the general practice to use a spectrometer in which a plurality of ions are formed from the molecules of different gases and vapors in an unknown mixture. After the formation of a considerable number of these ions, an electrical force is applied to the spectrometer to produce a movement of the ions from their place of origin toward a plate or measuring surface. The various ions present will be accelerated to a greater or less extent depending on their mass. The ions of light mass will have a greater velocity imparted to them than will the ions of a heavier mass.

In so-called time-o f-fiight instruments, this variation in velocity Will cause the lighter particles to strike the measuring surface before the ions of a heavier mass or weight. By the use of appropriate measuring means, the time of flight or travel of the various ions is measured thereby giving an indication of the mass'of the ions being received on the detecting plate or measuring surface. By the use of this technique, the mass of different molecules of gas in an unknown mixture can be determined in relationship to the flight time of the particles.

These types of mass spectrometers have undesirable characteristics, however, which are not easily overcome using conventional construction means. For example, some means must be incorporated into the mass spectrometer to provide for the correction of any difference in" the position of individual ions resulting from the finite radial width of the various elements within the circuit. This is particularly true for differences in the position and random motion of individual ions. Various attempts have been made in the past to correct for this difference in ion position without any marked degree of success.

The mass spectrometer thus far described is usually of a linear configuration and requires special construction considerations to insure any degree of accuracy in the measurements. This special construction obviously results in an increase in construction coast the finished product. The use of this type mass spectrometeralso requires that a very accurate pulsing circuit be used to impart accelera tion to the ions because, since measurement of the ion mass is dependent on the exact measurement of a finite flight time, any variations in the impulse circuit will result in an erroneous result being obtained. This type of mass spectrometer also requires that a very accurate and sensitive measuring circuit be provided. This is true because the ion bursts occur over relatively short periods of time thereby giving a pulsed output which adds to the difiiculty of obtaining a correct measurement.

The previously known and used mass spectrometers of the flight time type ordinarily did not measure theipresence' of one particular type ion without also recording signals produced by other type ions that might also be present in the spectrometer. It'is possible, but diflicult, to construct a circuit which will reliably detect and record an ion of only a given mass while excluding ions of any other mass that might be present within the spectrometer.

The most common type of mass spectrometers-are the magnetic deflection instruments. In these spectrometers,

3,lh2,76 Patented Dem 196 4 the ions are focused into a narrow beam by means of slit systems and electrical and/ormagnetic focusing. All of the ions are accelerated through the same electrical potential difference so that they have approximately the same energy. Ions of different masses, however, will have different momenta. When this beam. of ions is passed through auniform, homogeneous magnetic field, the ions are deflected into circular arcrlike. trajectories, the radius of each ion path being proportional to the momentum of that ion. Ions of the same mass which pass through the entrance slit of the instrument are. thus deflected by the magnetic field, separated from the ions of other masses, and finally are refocused through a collector slit onto the ion detector. By varyingthe accelerating potential or the strength of themagnetic field, various ion masses can be scanned or detected. 1

Magnetic mass spectrometers usually have ion sources which are fairly complex,.an d which must bevery carefully machined, assembled and aligned. The magnets used with such instruments are ordinarily quite large and heavy. High accelerating voltages of theorder of several thousand volts are required to obtain well defined ion beams. The instruments have dimensions of the order of one, two ormore feet and acorrespondingly large weight. r

According to the present invention, it has been found that these diiiiculties may be overcome by the use of a magnetron type mass spectrometer. The magnetron type mass spectrometer of the present invention is as simple in its principle of operation and almost as simple in its mode of construction as. the basic magnetron tube. Specifically, a mass spectrometer is described having a nonpl anar configuration and one, two or more diilerent ion collector plates or surfaces. By utilizing such configurations, advantages are obtained in that no critical homogeneous magnetic field is required nor any complicated electronic circuits. The slit system of focusing and collimation of ions commonly used with magnetic mass spectrometers has been eliminated, as has the use of the flight time principle for determining the mass of the various ions. This simplification of construction allows a lighter, smaller mass spectrometer having a high degree of sensitivity to be producedat a low cost.

An object of this invention is to provide a magnetron type mass spectrometer for determining the masses of diiterent ions by the use of one, two or more collector plates. or surfaces.

Another object is to provide a mass spectrometer which is simple in construction, 10W in cost and which requires no critical homogeneous magnetic field or electricalcircuitry.

Yet another object of this invention is to provide a mass spectrometer having the above characteristics but still maintaining a high degree of sensitivity.

Still another object of the invention is to provide a mass spectrometer that can be used as a pressure gauge to measure very low partial pressure.

It is a still further object'of the invention to construct a mass spectrometer which, while capable of attaining all of the above-described objectives, requires aminimum of space and is extremely light, which makes possible its disposition in practically all environments so that it possesses practically universal applicability.

These and further objects and advantages of the present invention will be come more apparent upon reference to the following description and claims and the appended drawings wherein:

FIGURE 1 is a sectional side view of a mass spectrometer constructed in accordance with the principle of this invention; 7

FIGURE 2 is a sectional top view of the mass spectrometer shown in FIGURE 1;

FIGURE 3 is a sectional side view of a mass spectrometer constructed in accordance with the principle of this invention which utilizes two grids;

FIGURE 4 is a sectional top view of the mass spectrometer shown in FIGURE 3;

FIGURE 5 is a sectional side view of another type mass spectrometer constructed in accordance with this invention in which the ions are focused within a box-like structure;

FIGURE 6 is a sectional top view of the mass spectrometer shown in FIGURE 5; 7

FIGURE 7 is a cross-sectional view of a mass spectrometer constructed in accordance with this invention and showing one of the various arrangements the ion collection may assume;

FIGURE 8 is a sectional side view showing a mass spectrometer for detecting light and heavy ions; and

FIGURE 9 is a sectional top view of the mass spectrometer shown in FIGURE 8.

The same reference numerals denote the same parts throughout the several views of the drawings.

With reference to FIGURE 1, the mass spectrometer is shown generally as 10 and consists of an airtight housing or casing 12 which may be constructed of glass, metal or other suitable material. Contained within the housing or casing 12 is a filament structure 14. The filament 14 may be made of any suitable material, such as tungsten, which is adapted to emit electrons when heated. Surrounding the filament 14 is a grid-like structure 16 which serves the dual function of accelerating and collecting the electrons emitted by the hot filament 14 and accelerating the positive ions in. the. region between the grid 16 andthe ion collectors18and 20. Located radially outward from the filament and grid. structure is an innef collector plate or surface 18. An outer collector plate or surface 20 is shown positioned. a greater distance from the filament and grid structures. The collector plates or surfaces 18 and 20. are generally formed in the shape of a semi-circular configuration as more clearly shown in FIGURE 2.

An extension tube 22 is provided for connectingthe. mass. spectrometer to a, container which is to be tested or which contains the various gaseous or vaporous materials to be tested. The filament 14 and grid 16 are shown connected to an electron and. ion accelerating power supply 24. A permanent magnet. 26 is shown for supplying a magnetic field indicated by the arrow B to the mass spectrometer.

The semi-circular plates or collectors 18 and 20. are shown connected to separate direct current sensors 28 and 30, respectively. Ion current balancing circuits 32 and 34 are also shown connected to the sensors 28 and 30, respectively, to assure that the sensors may be. properly balanced so that when ions of all masses strike the two collectors, equal ion currents will flow in the two collectors. The difference in readings between the two sensors indicates the ion current produced by the selected ions that are collected only by the inner collector 18. If desirable, a differential direct current meter could be connected between the two plates to measure this same difference current.

The filament current is shown as being adjustable by means 36 so that the sensitivity of the spectrometer may be varied. Thus, various ion masses are focused on the inner collector 18 by varying the grid' and filament potentials.

In reference to FIGURE 2, which is a cross-sectional view of the spectrometer, the configuration of the plates or collectors 18 and 20 is more clearly shown and it may be readily seen that the radius of the collector 18 is less than that of the collector 20. The numerals 38-44. are used to indicate generally the movement of various ions having different weights or masses.

The operation of the magnetron type mass spectrometer can best be understood by first considering the operation of a simple magnetron tube. The simplest type magnetron tube is a diode having a circular outer electrode (anode) and a coaxial center wire electrode (cathode). The outer electrode is made electrically positive with respect to the center wire. In operation the center wire is heated so as to emit electrons thermionically and these electrons are accelerated toward the outer anode. If the diode is placed in a uniform magnetic field which is directed axially along the tube, the electrons are caused to deviate from their normal radial motion. If this magnetic field is made sufficiently strong, the electrons are prevented from reaching the anode and hence no current will fiow to the magnetron tube. Thus, this simple type magnetron tube may be used as a current switch.

The minimum value of the magnetic field which interrupts a current flow in a simple magnetron is known as the cutoff or critical magnetic flux density. The value of this flux density is given by the following expression:

The symbols above represent the following quantities: B =critical magnetic flux density r =radius of the outer electrode (anode) r =radius of the inner electrode (cathode) a potential difference betwen cathode and anode m =mass of the electrons =charge of the electrons The above equation is derived on the basis of certain simplifying assumptions, one, of which is that all the electrons leave the. cathode. with zero initial velocity. If found desirable, it is possible to modify the equation to take into account most of the physical realities of the situation.

The magnetic type mass spectrometer described in this application will operate. in much the same manner as the elementary magnetron just described. The principal difference between the elementary magnetron and the mass spectrometer is found in the substitution of positive ions for electrons. These positive ions to be studied are produced by a portion of a gaseous or vapor sample being actedupon by a beam of electrons. The impact of the electrons. against the particles (atoms or molecules) of the sample givesrise to the desired positive ions.

The. positive ions used in this invention are created by a. sample of the material under study entering the area of the filament 14. Here within the area of the filament, the sample, is subject to electron bombardment so as to produce a. cloud of positive ion particles. These particles are then accelerated radially outward toward the outer plates of collector electrodes 18- and 20.

In reference to FIGURE 2, the particle flight of various. ions is shown in greater detail. If no magnetic field were: created along the axis of the magnetron mass spectrom-- eter, the. normal flight or zeromagnetic field ion path of a. positive ion, which has been accelerated by a potential applied tothe grid 16, would be in a radial outward direction as indicated by arrow 38. Thus, all particles regardless of their mass would strike both plates 18 and 20. Under these conditions, no reading or indication as to whether or. not a particular ion of a given mass were present in the sample under study could be obtained. The application of an axial magnetic field to the tube, however, will cause thepositive ions to assume a curved flight path which will eventually result in particles falling into a curvilinear orbit if they do not strike the collectors 18. and 20. The amount by which a particle is deflected into a curvilinear path is dependent primarily upon the mass of the individual particle, with the particle having a lighter mass being deflected to a greater degree than a particle having a greater mass.

From the above, it may be seen that the magnetic field B created. by the permanent magnet 26 or an equivalent electromagnet, and the potential difference between collec tors and cathode, e can. be adjusted so that all ions up to a given critical mass are cut off from striking either collector 18 or collector 20. The influence of the magnetic field upon various ions having different masses is more clearly shown by the flight paths 40-44 as shown in FIGURE 2.

Assume that the magnetic field and collector to cathode potentials a (the potential on collector 18) and e (the potential on collector 26) of the magnetron mass spectrometer are adjusted to detect the presence of a particle such as, for example, helium. Under these set conditions, any helium ions that are present will be accelerated through the grid 16 and will fall into a curved flight path such as indicated by the curved line. 40. As can be seen by this line, the amount that the helium particle is deflected or curved will be such as to cause the flight path of the particle to intersect or strike the collector plate 18 which is located at a radial distance r from the cathode. None of the helium ion particles will strike the collector 2%, however, even though the particles are initially accelerated toward. this collector. This is true because the collector 20 is located at a greater radial distance r from the cathode and the helium ions will have gone into a curvilinear orbit before they reach this collector.

If ions having a mass less than the ions which are to be detected are present in the sample under test, they will be cut oif from reaching either collector 1% or 20. The flight path of an ion having a lighter mass than helium is shown as curved line 42. As explained above, the lighter the mass of the particle, the greater amount it is deflected by a given axial magnetic field. Thus, with the magnetron mass spectrometer adjusted to detect the presence of helium, the magnetic field exertedoy the permanent magnet or its equivalent electromagnet will be of such strength as to deflect any lighter particle into an orbital path which will be of insufficient radius to strike either collector, as more clearly shown by line 42.

Ions having heavier mass than the ions for which the spectrometer is set to detect are shown as curved lines 44. As stated above, ions having heavier mass are deflected to a less degree than ions of a lighter mass. Thus, the heavier ions represented by lines 44 are not deflected to as great an extent as are the lighter ions represented by lines 4tl-42. From this it may be seen that ions having a heavier mass than those for which the spectrometer was set to detect will strike both collectors 18 and 20 equally.

With both collectors 18 and 20 receiving the same number of heavier ions and with each ion generating the same degree of electrical energy, it can be seen that the signals produced by these heavy ions when fed from the collector plates 18 and 20 into a measuring system, such as indicated by sensors 28 and 36 will effectively produce the same reading on each sensor. With the signal produced by the heavier ions striking both plates 18 and Zli equally, any difference in the reading produced by the sensors will be due solely to the selected ions, represented by the curved line 40, striking the collector plate 13.

In FIGURES 3 and 4, a second embodiment of a magnetron type mass spectrometer is shown which utilizes two grids instead of one. The innermost grid In is adiacent to the filament and serves to accelerate and collect the electrons emitted by the filament. Positive ions which are formed near this electron grid are then accelerated by the second, outermost grid 46 and caused to enter the region between this latter grid and the ion collector plates with high initial velocities. In the particular arrangement shown, the grid 46 has curved-over edge portions 47 which extend substantially to the collector plates. The accelerating grid 46 and the collector plates 13 and 2d are maintained at or near ground or zero potential so that the region between the accelerating grid and the ion collectors has no electric field. Thus the high velocity ions entering. this region are acted on only by the force below thepotential of the electron accelerating grid 16 while the filament has a potential thatis 25 to volts less than that of the electron accelerating grid. In this case, most of the positive ions will be formed in the region between the two grids and will. be accelerated from this region into the high'electric field region between grid 46 and the ion collectors 18 and 2h.

The mass resolution for this second embodiment is substantially better than that for the first embodiment shown in FIGURES 1 and 2. In addition, this second embodiment permits greater flexibility in the choice of the operating parameters. Various ion masses can be focused on the inner ion collector by keeping the potentials constant and varying the axial magnetic field strength B as well as by keeping the magnetic field strength constant and varying the electrode potentials.

FIGURES 5 and 6 show a third embodiment of this mass spectrometer. The difference between this embodiment and that of FIGURES 3 and 4 is the dilferent way in which positive ions are created. In this third arrangement, the ions are formed within a centrally located, completely enclosed metallic box 52. This box, called the ionization region enclosure, has aslit 54 at one end where electrons from a heated filament 14 may enter the enclosure. Similarly, there is a matching slit 56 at the other end of the box where electrons may leave the enclosure and be collected by an electron collector electrode 58. There is almost no electric field within the box, that is, it is a region having an approximately constant potential. The cylindrical sides 60 of this box are perforated or otherwise covered with a grid-like gauge or wire mesh so that neutral gas molecules can enter the box freely. Positive ions that are created within the box can diffuse or drift through the perforations or gridlike structure of the sides and enter the acceleration region between this ionization region enclosure 52 and the ion accelerating grid 46. These ions are then accelerated into the region between the ion accelerating grid 46 and the ion collectors 18 and 20.

As in the second embodiment of FIGURES 3 and 4, there is a choice of operating the ion accelerating grid 46 at "or near the collector potentials (which are near zero potential), or at a high positive potential which is sufficiently less than the ionization region enclosure 52 potential so as to extract ions therefrom. Additional screening and shielding electrodes can be used to make the interior of the ionization region enclosure 52 more of an equipotential (zero electric field) region. The ion accelerating grid, when operated at a potential near the collector potential, can be made slightly negative with respect tothe ion collectors so that it may more readily collect ions that do not strike the collectors and thus prevent a positive space charge from building up in this region. The prevention of the accumulation of space charge in the region adjacent to the ion collector plates will insure better performance of all embodiments of the magnetron type mass spectrometer.

It should be noted that the electrons flow in the direction of the magnetic field, that is, from the filament 14 to the electron collector electrode 53. Electron motions perpendicular to the magnetic field are thus inhibited. In this way, the electron beam will be well-collimated so that passes easily through the slits at the ends of the ionization region enclosure and is confined to the central portion of the tube. The results of this collimation are highly desirable for the following reasons. First, the electron beam current will be increased so that the spectrometer sensitivity will be increased and, second, the crea tion of X-rays caused by electrons striking the walls of the ionization region enclosure will be minimized.

The elimination of any X-rays is a primary concern where the spectrometer is to operate in a very low pressure range such as, for example, 10- mm. of mercury. This is true because the X-rays irradiate the ion collectors and cause the ejection of photoelectrons therefrom. These photoelectrons leaving the ion collectors are indistinguishable from positive ions arriving there. The X-ray photoelectric current thus sets the limit for the lowest practical gas pressure (lowest ion concentration of a particular spectrum) that can be measured.

Another chief advantage of the mass spectrometer design shown in FIGURES and 6 is the creation of all positive ions within an approximately equipotential (zero electric field-or electric field free) region. Because of this, all positive ions will receive approximately the same acceleration when they leave this equipotential region and move toward the ion collectors. The ion mass resolution of magnetron type mass spectrometers depends on the uniformity with which ions are accelerated. The largest ion mass M (atomic mass number) which can be completely separated from its neighboring ion mass (M-l) is given by the equation:

where V is the average acceleration voltage in volts and AV is the maximum variation in the acceleration (expressed in volts) received by the positive ions. For example, if the average acceleration voltage V is 500 volts and AV is 25 volts, then we should expect to completely separate mass 21 from mass 20. r

In the spectrometer design shown in FIGURES 5 and 6, the maximum variation in the acceleration voltage AV will be small so that the mass resolution for this arrangement will be high. To achieve even higher mass resolutions, additional shielding electrodes may be introduced to prevent the penetration of outside electric fields into the interior of the ionization region enclosure 52 so that this region becomes more and more of a perfect equipotential region.

In addition to the three methods of producing ions that are specifically described and illustrated, other methods of creating positive ions in the central part of the mass spectrometer can be utilized. Among these methods would be surface ionization on a high temperature heated surface, ionization by the creation of a low pressure discharge (Penning type discharge) in the magnetic field a1 ready present, and ionization by use of high energy photons (ultra-violet light, X-rays) or radiations from radioactive substances (alpha, beta or gamma radiations).

The design of magnetron type mass spectrometers is not limited to the use of two ion collectors shaped in semicircular fashion as shown in FIGURES 1 through 6. Other ion collector arrangements are possible and practicable. One such ion collector arrangement that can be used is illustrated in FIGURE 7. This arrangement consists of dividing each ion collector into segments of approximately equal size and placing them about all or a portion of the tube perimeter. The segments of the inner ion collector 18 more or less cover the area not covered by the'segments of the. outer ion collector 20. By rotating one of these multisegment collectors about the tube axis, so that it moves relative to the other ion collector, the ion currents to the two collectors can be balanced for those ion masses that are larger than the ion mass to be detected. This rotating of one of these multisegment collector arrangements could be accomplished by mounting the collectors on a movable ring within the housings (not shown).

Another arrangement of ion collectors that may be used in this invention is shown in FIGURES 8 and 9. In addition to the One or more heavy ion collectors 18 and 20 that are located near the periphery of the tube, as already described, a plurality of lighter ion collectors 6646 may be located near the center part of the tube just outside the region where the positive ions are formed. The light ion collectors 6066 are shielded by shielding means 68-74, respectively, so that they cannot directly collect the positive ions that are moving radially outward from the central ionization region.

A light ion of a proper mass is collected by first being permitted to pass through one of the gaps between the light ion collectors. Such a selected ion is represented by the broken lines 76.

If the ion moves in the proper curvilinear trajectory, it will not strike the heavy ion collector but will start moving radially inward and will strike the light ion collector, as shown by line 76. Ions heavier than this particular selected mass will strike the heavy ion collectors 18 and 20. Ions lighter than the selected mass will either strike the light ion collector shields or will be returned to the region of positive ion formation. These lighter ions are represented by broken line 78.

One of the advantages of the light ion collector arrangement is that in this case the desired (selected mass) ion current is measured directly and not as a difference current. Only ions of the proper mass strike the collectors 60-66 so that the operation of the spectrometer is independent of the presence of ions of the other masses. Another big advantage of this arrangement is the fact that all ions heavier than the selected ions are collected by the heavy ion collectors 18 and 20 and neutralized so that they do not contribute appreciably to the production of space charge. Another advantage is the presence of a shield between the light ion collector and the region where X-rays originate. Because of this, very small ion'currents can be measured.

The light ion collector arrangement has proven ideal for the measurement of light gases such as hydrogen and helium. The relatively large difference in the masses of atomic hydrogen, molecular hydrogen, helium and all other constituents of normal air make it easy to separate and resolve these light ions from each other and from all the heavier ions.

It is obvious that many variations of this basic mode of operation are possible and will be readily apparent to those skilled in the art. For example, a magnetron type mass spectrometer may be built with any combination of accelerating grids and ion collector arrangements so that two or more desired ion species may be measured simultaneously. Thus it may be possible under certain conditions to use more than two collector plates to detect two or more ion species simultaneously.

The sensitivity of the spectrometers disclosed may be increased by making the ionization region relatively larger so that most of the ions formed could be detected. Any loss due to axial ion drift and scattering can be minimized by proper location of the electrodes. Shorter ion paths will minimize scattering.

Any positive ion space charge which may be present in the tube will not affect the operation of the system because, even though the flight path of the various ions may be comewhat altered, the maximum radii attained by the ions will not change. This fact is borne out by the above equation which is independent of any particular potential distribution between the anode and cathode.

The mass spectrometer design illustrated in FIGURES 1 and 2 above has been found especially useful in detecting trace quantities of heavy elements in predominantly light. gases. It should be noted that the mass spectrometer design of FIGURE 8 will do the opposite, such as detecting small quantities of helium or hydrogen in a heavier gaseous material. A preliminary study has shown that a small tube having a diameter of two inches and a magnetic field of 2,000 oerstedsshouldbe able to resolve completely all. masses up-to about. a mass of 50 atomic mass units. Sucha tubeis extremely useful in analyzing atmospheric components such'as helium, hydrogen, water vapor, nitrogen, oxygen and so forth It. will be apparent from the foregoing that the device of this invention is simpler in construction than heretofore known mass spectrometer tubes or analyzers and therefore may beproduced at a lower cost. The absence of any criticalhomogeneous magnetic, field or complicated electronic circuitry, also contributes to the reduction in cost. The small size and light weight of the spectrometer will allow the device to lend itself to anyrprogram involving use in a system where space is ata premium, such as rocket-borne or satellite-borne detection systems. The fact that the spectrometer may also-be used as a pressure gauge to measure veryv low particle pressures adds to its versatility and range of uses.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. A magnetron mass spectrometer, including a housing, a filament adapted to produce a source of electrons located within, said housing, ion acceleration means located near said filament, a first semi-circular collector located soas to partially surround said filament, a second semi-circular collector located so as to partially surround said filament but located a greater distance from said filament than said first collector, electrical lead-out means connected to each of saidcollectors, means for inserting a material-to be tested into the region of said filament, means for producing a magnetic field axial to said collectors so that ions of a given mass produced by said material being acted upon by the electrons from said filament will be accelerated by said ion accelerator means and will strike only the said first collector, circuitry means connectedto said electrical lead-outs to detect said ionsstriking. said first collector, and circuit means con nected to .said filament and said collectors for. varying the various ions focused on said collectors.

2. A magnetron mass spectrometer according to claim 1 wherein said ion accelerator means comprises a first grid structure substantially surrounding said filament.

3. A magnetron mass'spectrometer according to claim 1 wherein-said ionaccelerator means consists of a first grid structure substantially surrounding said filament, and a second grid structure substantially surrounding said first grid structure, said second grid structure having edges which extend substantially to said first and second collectors whereby. a region is established having sub? stantially no electrical field.

4. A magnetron mass spectrometer, including a housing, means for producing ions comprising a grid-like metalliclbox located within said housing, said box having an opening ateachend, filament means located near one opening of said box for. producing electrons, electron collector means located near theother opening of said boxfor receiving the electrons produced by said filament, means for connecting said filament to a variable source of current and potential, means for connecting said electron collector means to a variable source of potential, first semi-circular collectormeans located so as to partially surround said metallic box, second semi-circular collector means located so as to partially surround said metallic box, saidsecondcollector being located a greater distance from said metallic box than said. first collector, means for connecting said first and second collectors to ion indicating means, ion accelerator means located between said metallic box and said first and second collector means, said ion accelerator means consisting of. a circular U- shaped grid, the edges of said LI -shaped grid extending substantially to said first and second collector means whereby a region is formed between said grid and said first and second collectors which is not affected by external electrical disturbances, means for connecting said grid to a variable source of potential, means for pro ducing a magnetic field axial to said first and said second collectors, and means. for insertijng a material. to be tested into the region of said metallic box whereby the ions of. a given mass produced by said electrons will be accelerated and detected by said first and second collector means. 7

5. A magnetron mass spectrometer according to claim 4- wherein saidfirst and second collectors are divided into a plurality of segments, said segments of said first and second collectors being alternately arranged so that the segments of said first collector substantially cover the area not covered by the segments of said second collector.

6. Amagnetron mass spectrometer, including a housing, a filament adapted to produce a source of electrons located within said housing, a first semi-circular collector located so as to partially surround said filament, a second semifcircular collector located so as topartially surround said filament but a greater distance from said filament than said first collector, a grid structure located between said first and second collectors and said filament, a third semi-circular collector located between said first and second collectors and said grid structure, shield means located between said third collector and said grid structure, means for connecting said collectors to ion indicating means, means for inserting a material to be tested into the region of said filament, and means for producing a magnetic field axial to said collectors whereby the ions produced by said material being acted upon by the electrons from said filament will be accelerated by said grid structure with ions of a certain mass continuing radially outward and striking said first and second semi-circular collectors while ions of a certain mass will move radially inward and strike said third collector, said third collector being shielded from ions moving radially outward by said shielding means. I

7. A magnetron mass spectrometer comprising a source of ionparticles, two collectors disposed about said source, each of said collectors having an inner surface all points on which are equidistant from said ion source, one of said collectors being spaced farther from said source than the other, means for accelerating said ion particles from said source radially toward said collector, means for balancing currents to each of said-collectors due to ions of greater than a given mass, and means for detecting ion currents due to ions of said given mass to the collector closer to said source.

8. A magnetron mass spectrometer comprising a source of ion particles, two collectors disposed concentrically about saidsource, each of said collectors having an inner surface all points on which are equidistant from said ion source, one of said collectors being spaced closer to said source than the other and having at least an open ing formed therein for the passage of ions, means for accelerating said ion particles from said source radially toward saidcollector, means for balancing currents to each of said collectors due to ions of greater than a given mass, and means for detecting ion currents due to ions ofsaid given mass to the collector closer to said source.

9. Inv a mass spectrometer-having the general configuratiori of a magnetron with a source of ions of various masses disposed centrally thereof and means disposed about said source for accelerating said ions in generally radial paths, the combination of a first ion collector disposed concentrically about said source at a first distance heref o a 1 i9 H Ct r disposed concentrically about said source at a second distance greater than said first distance therefrom, means for applying a first potential between said first ion collector and said source to produce a first electric field therebetween, means for applying a second potential between said second ion collector and said source to produce a second electric field therebetween, means disposed about said spectrometer for producing a magnetic field therein, and means for "adjusting the relative strength of said fields one to another to cause ions of a mass greater than a predetermined mass to impinge on both of said ion collectors, ions of a mass less than said predetermined mass to impinge on neither of said ion collectors, and ions of said predetermined mass to impinge only on said first ion collector.

10. In a mass spectrometer having the general configuration of a magnetron, a centrally disposed electron source and means for introducing a gaseous sample into said spectrometer, the combination of means for bombarding said gaseous sample with electrons from said source to produce ions of various masses, an accelerating grid disposed about said source for causing said ions to move in generally radial paths from said source, a first ion collector disposed generally concentrically about said accelerating grid, a second ion collector disposed generally concentrically about said first ion collector, at least one opening being formed in said first ion collector, means for applying a potential between said first ion collector and said source to provide a first electric field therebetween, means for applying a potential between said second ion collector and said source to provide a second electric field therebetween, means for providing a magnetic field between said source and said ion collectors, and means for adjusting the relative strength of said fields to cause ions of a mass less than a predetermined mass to depart from radial paths and assume circular orbits of diameter less than the distance from said accelerating grid to said first ion collector, ions of a mass greater than said predetermined mass to continue in radial paths to both of said ion collectors, and ions of said predetermined mass to reach only said first ion collector.

11. A magnetron mass spectrometer comprising a gen erally cylindrical housing, means for establishing a magnetic field axially of said housing, a source of ions dis posed generally centrally of said housing, a first collector disposed about said source, said first collector having a surface all points of which are at a first distance from said source, a second collector disposed about said source, said second collector having a surface all points of which are at a second distance greater than said first distance from said source, at least an opening being formed in said first collector, means for accelerating ions from said source radially toward said collectors, means for balancing ion currents to said first and second collectors due to ions of greater than a given mass, and means for detecting ion currents to said first collector due to ions of said given mass.

12. A magnetron mass spectrometer comprising a generally cylindrical housing, means for establishing a magnetic field axially of said housing, a source of ions disposed generally centrally of said housing, a first collector disposed about said source, said first collector having a surface all points of which are at a first distance from said source, a second collector disposed about said source,

said second collector having a surface all points of which are at a second distance greater than said first distance from said source, at least an opening being formed in said first collector, means for accelerating ions from said source radially toward said collectors, said ions being selectively deflected according to their mass into curved paths of varying radii by said axial magnetic field, means for modifying said magnetic field to balance ion currents to said first and second collectors due to ions of greater mass than a given mass, and means for detecting ion currents tosaid first collector due to ions of said given mass.

13. A magnetron mass spectrometer comprising a generally cylindrical housing, means for establishing a magnetic field axially of said housing, a source of ions disposed generally centrally of said housing, a first collector disposed about said source, said first collector having a surface all points of which are at a first distance from said source, a second collector disposed about said source, said second collector having a surface all points of which are at a second distance greater than said first distance from said source, at least an opening being formed in said first collector, means for establishing electric fields between said source and said collectors to cause ions to move generally radially from said source toward said collectors, said ions being caused to selectively depart from said radial movement in accordance with their mass by said axial magnetic field, means for modifying said electric fields to balance ion currents to said first and second collectors due to ions of greater than a given mass and means for detecting ion currents to said first collector due to ions of said given mass.

14. In a magnetron mass spectrometer having the general configuration of a magnetron including .a generally cylindrical housing, means for establishing a magnetic field axially of said housing, and a centrally disposed source of electrons, the combination of a plurality of ion collectors disposed within said housing at different distances from said source of electrons, and a box-like structure at least partially surrounding said source of electrons, said box-like structure having openings formed therein for the passage therethrough of ions and electrons.

15. In a magnetron mass spectrometer having the general configuration of a magnetron including a generally cylindrical housing, means for establishing a magnetic field axially of said housing, and a centrally disposed source of electrons, the combination of an enclosure disposed at least partially about said source of electrons, means for passing said electrons through said enclosure, openings being formed in said enclosure for the introduction of gas molecules, ions being generated by collisions of said electrons and said molecules, a plurality of armately formedv collectors, each collector having at least a surface all points of which are equidistant from said enclosure, and grid structures disposed about said enclosures to accelerate said ions toward said collectors.

16. In a magnetron mass spectrometer having the general configuration of a magnetron including a generally cylindrical housing, means for establishing a magnetic field axially of said housing, and a centrally disposed source of electrons, the combination of a plurality of generally semicircular heavy ion collectors disposed within said housing at different distances from said source of electrons, alight ion collector disposed closer to said source of electrons than said heavy ion collectors, an enclosure formed at 'least partially about said source of electrons, means "for introducing gas molecules into said enclosure for the generation of ions by collisions of said molecules with said electrons, means for accelerating ions generally radially outward from said enclosure, and shield means disposed between said light ion collector and said enclosure to prevent direct impact on said light ion collector by ions moving generally radially from said enclosure.

17. In a magnetron mass spectrometer having a cylindrical housing, a centrally disposed source of electrons, and a magnetic field established axially of said housing, the combination of an ion-generating chamber disposed generally about said source of electrons, a plurality of coaxially disposed arcuate ion collectors, each said collector having at least a surface all points of which are equidistant from said chamber, and each collector being at a different distance from said chamber and means for accelerating ions from said chamber to strike said surfaces approximately tangentially.

18. In a magnetron mass spectrometer as defined in claim 17, at least one ion collector being in generally cylindrical formed segments, each said segment having a surface forming a part of said surface all points of which are equidistant from said chamber.

19. In a magnetron mass spectrometer having a cylindrical housing, a centrally disposed source of electrons, and an axially disposed magnetic field in said housing, the combination of an enclosure, means for introducing gas molecules into such enclosure, means for passing electrons through said enclosure to generate ions by collision of said molecules therewith, arcuate collectors disposed coaxially within said housing at different distances from said enclosure, grid means for accelerating said ions toward said collectors and means for detecting the difference in currents due to ion flow to said collectors.

References Cited by the Examiner UNITED STATES PATENTS Fritz 3l3157 Bennett 25041.9 Washburn et al 25041.9 Lanneau et al. 2504l.9

Barnes 250-419 McNarry et al 250--41.9

RALPH G. NILSON, Primary Examiner.

ARTHUR GAUSS, Examiner. 

7. A MAGNETRON MASS SPECTROMETER COMPRISING A SOURCE OF ION PARTICLES, TWO COLLECTORS DISPOSED ABOUT SAID SOURCE, EACH OF SAID COLLECTORS HAVING AN INNER SURFACE ALL POINTS ON WHICH ARE EQUIDISTANT FROM SAID ION SOURCE, ONE OF SAID COLLECTORS BEING SPACED FARTHER FROM SAID SOURCE THAN THE OTHER, MEANS FOR ACCELERATING SAID ION PARTICLES FROM SAID SOURCE RADIALLY TOWARD SAID COLLECTOR, MEANS FOR BALANCING CURRENTS TO EACH OF SAID COLLECTORS DUE TO IONS OF GREATER THAN A GIVEN MASS, AND MEANS FOR DETECTING ION CURRENTS DUE TO IONS OF SAID GIVEN MASS TO THE COLLECTOR CLOSER TO SAID SOURCE. 